Noisy HDD in Idle

Hi, 

I have a Raidz2 with 4x Toshiba N300 16 TB drives. 
As soon as I add the 4th drive and resilvering has completed or shortly after the drive or one of the drives becomes super noisy even when idle. It sounds like a lot of data is being written but the system is completely idle. 

Truenas Scale is running as a proxmox VM with a HBA card in a Dell T5820. 

I tried calling Toshiba just now but they just said to maybe give it a week or two. 

Does anyone have an idea what could cause this?

As soon as I remove the 4th drive the server becomes silent again. This is also already the third replacement drive. 

By the way if I remove the "broken" drive and insert it into my desktop PC it also makes this noise even though it's not mounted. Smart values look okay to me. 
https://pastebin.com/ge9htrQY

Here is a short recording: https://whyp.it/tracks/186087/hdd-noise?token=kRiQ0

the disks are in the front Hot Bays. I have three SSDs inside the case. Inside there was only one additional Sata power cable so I used a splitter to attach them. The PSU has 950 watts. This shouldn't cause any issues with the HDDs right? Just wanted to add this since it's the only thing that's "special"